I had the problem, that I was not able to see my backup history with Time-Machine even so they show up when using finder. I have not found out why that happens, but the following suggestion allowed me to access them directly:
This suggestion was found here.
Apple changed something so Time Machine now thinks you've got a different computer. All your backups are there, as you can see via the Finder, but TM has started a new, separate sequence.
Select the TM icon in the menubar and hold your mouse down. Watch the +Enter Time Machine+ option while pressing the Option key on your keyboard. That option should change to +Browse Other Time Machine Disks+. Select it and you'll see your previous backups.
You cannot merge the two sets of backups, as TM thinks they're for different computers, but you can restore files/folders from them if you need to.
This suggestion was found here.
